Study of the response of miniature fission chambers in fluctuation mode (PhD)

In a nuclear reactor the on-line neutron flux measurement is often realized by means of fission chambers.  The choice of the fissile deposit in this kind of detectors depends on the neutron energy range of interest. The resulting on-line spectral information is valuable for power reactor surveillance, for neutronic studies at zero power reactors and for flux monitoring in experimental devices in a materials testing reactor.

A fission chamber is typically composed of two coaxial cylindrical electrodes (with an external diameter of a few mm for in-core detectors), one of which is covered with fissile material.  The inter-electrode gap is filled with gas, often argon.  After a neutron induced fission in the deposit, one of the fission products is ejected into the gas in which it creates a large number of charge pairs.  These charges are collected by a polarization voltage applied between the electrodes, leading to a current pulse.   For low fission rates, these pulses can be counted separately, providing a measure of the neutron flux (pulse mode operation).  For moderate or high fission rates, the individual pulses are piled up.  In that case two other operation modes can be applied: the current mode uses the mean delivered current as a measure for the neutron flux, while in the fluctuation mode the variance of the current is measured.  The main interest of the latter method is the suppression of gamma-induced signal components relative to the neutron induced fission related signal.  This is a consequence of the fact that the variance of the current is proportional to the square of the collected charge per event, while the mean of the current scales with the charge itself (Campbell theorem).

The fission chamber "sensitivity" (the ratio between the detector signal and the neutron flux) is often determined experimentally in a well characterized reference neutron field. However, this approach does not guarantee the applicability of the obtained sensitivity value in all circumstances: many parameters (neutron spectrum, neutron-gamma ratio, temperature, polarization voltage, environment,…) should be taken into account for a reliable conversion of the signal into the desired neutron flux information.  Therefore, it is necessary to understand the details of the mechanisms leading to the signal in order to be able to extrapolate the experimental sensitivity to the operation conditions, or, even better, to calculate the sensitivity ab initio.

The calculation of the sensitivity of a fission chamber requires the development of numerical simulation tools, which are also very useful in the optimization of the detector design.  Meanwhile the simulation of fission chambers in current mode has reached a mature state, but for the fluctuation mode no advanced models exist.

The objective of the thesis work consists in the modelling of the fission chamber functioning in fluctuation mode which will allow to calculate the sensitivity.  The different physical phenomena leading to the detector signal will be studied and quantified:

- creation of the fission products and their transport out of the fissile deposit, taking into account neutron self-shielding and long-term evolution of the fissile deposit

- interaction of the fission products with the gas (ionization)

- recombination of the charge pairs and secondary ionization

- charge collection, taking into account the screening of the electric field by the space charge (as a function of the fission rate)

- generation of a fluctuating current by accumulation of pulses with emphasis on the evaluation of the variance of this signal taking into account the shape and the amplitude of the separate pulses.

This theoretical work will be complemented by experimental calibration tests in a research reactor and possibly by other experiments needed for validation of the developed models.

The thesis will be realized in the framework of the SCK•CEN-CEA Common Instrumentation Laboratory.  The theoretical work will be carried out in close collaboration with CEA.